The present invention is related to thermal disinfection or pasteurization of liquid sludge, such as sewage sludge. Disinfection is the inactivation of pathogens to levels low enough so that the sludge can be beneficially used, e.g. for land application, without infection risks. Pasteurization is the disinfection of liquids by heat. Thermal disinfection requires maintenance of every sludge particle at a certain temperature T (above 50xc2x0 C.) for a minimum time period t. The time period t depends on the temperature T, but should not be less than 30 minutes for sewage sludge, even if T is 70xc2x0 C. or above. The Environmental Protection Agency (EPA) of the United States of America has published time-temperature relationships for thermal sludge treatment in Biosolids Rule 503. The present invention proposes a continuous flow system for thermal sludge disinfection complying with the EPA regulations and providing so-called Class A Biosolids.
The invention is related to various process combinations for thermal disinfection and stabilization of liquid sludge, e.g. thermal pre-pasteurization followed by anaerobic digestion, aerobic-thermophilic pre-pasteurization followed by anaerobic digestion, or temperature-phased thermophilic/mesophilic anaerobic digestion. In the latter process combination, the system according to the present invention is provided between the thermophilic and mesophilic digestion stages, whereby sludge heating occurs prior to or as part of the thermophilic digestion stage.
Prior art knows batch systems for sludge pasteurization. Sludge is heated to a temperature T. After the temperature T is reached, batches of heated sludge are maintained in batch tanks at temperature T for a time period t. After the time period t has expired, sludge batches are removed from the tanks and cooled. If heat is provided continuously, a continuous sludge flow should be heated, e.g. in a heat exchanger. If a sludge flow is continuously heated, at least three batch tanks are required to provide the minimum residence time. While one tank is filled with heated sludge, the second tank is at rest providing the minimum batch detention time t, and the third tank is emptied. After every cycle with a duration of t, the tanks are exchanged: The filled first tank is now at rest, the second tank is now emptied, and the emptied third tank is now filled; and so on.
Such batch systems have several disadvantages. They require a complicated control system including at least 6 automatic valves and three level sensors. Synchronization of fill and draw pumps is difficult. There is an inherent danger of recontamination because each tank is periodically filled with not yet disinfected and therewith pathogen containing sludge. If only a little portion of the sludge in a tank, e.g. in a pipe connection or in the top of the tank, is not subject to the required temperature T for the required time t, the disinfection result might not be sufficient. While a tank is filled, air is displaced. The displaced air may not enter any of the other tanks because of the risk of recontamination. Therefore the air has to be vented to the atmosphere. This exhaust air is very odorous and requires deodorization. The airflow is the same as the sludge flow. Each tank must provide for the minimum detention time. The total volume of the three tanks is therefore V=3*t*q, whereby q is the sludge flow.
The present invention provides a pasteurization system with continuous flow. Sludge not only flows continuously through the heating means, but also through at least one chamber. These chamber or chambers provide for the required minimum residence time t at temperature T. No sludge particle is to leave the chambers before the minimum detention time t has expired. A single level sensor is sufficient to synchronize fill and draw pumps. Less automatic valves are required. Very little odorous air is displaced because the sludge level in the chamber is maintained almost constant and is changing only slowly dependent on the difference of the incoming and outgoing sludge flows.
A system according to the present invention comprises at least one chamber, however there can be more than one chambers, e.g. two or three chambers. The sludge from the sludge heater continually enters the first chamber. It can enter the chamber near its top or bottom. If it enters the first chamber at the top, it leaves the first chamber at the bottom, or vice versa. In this way, the sludge flows in a vertical, downward or upward direction through the first chamber. The same applies to all other chambers, whereby the flow through the last chamber has preferably a downward direction. The level in the last chamber is monitored and used to control the sludge withdrawal pump, i.e. synchronizing the sludge withdrawal flow with the sludge feed flow.
The new continuous flow system is easy to control and prevents recontamination by clearly separating a location where contaminated sludge enters the first chamber and another distant point where disinfected sludge leaves the last chamber. contaminated sludge is never in contact with disinfected sludge.
The flow through the chambers is slow; it is a so-called laminar flow. A laminar flow through a tube has a parabolic flow pattern. The velocity at the center of the tube is two times the average velocity, and the velocity at the wall is zero. The minimum residence time of all sludge particles is therefore half the mean residence time. The same applies to the flow through a non-circular chamber, whereby a hydraulic diameter is used instead of a real diameter. The hydraulic diameter is 4 times the cross sectional area divided by the perimeter. The mean detention time t2 in the chambers should be at least two times t. The fastest sludge particles have a detention time of minimum t. The total tank volume is V=2*q*t and is therewith only ⅔ of the total tank volume of a batch system.
The slimmer the chambers, the lower the risk of short-circuiting. A single chamber should have a height to width ratio of minimum 3. Where several chambers are used, the flow path length 1 through these chambers should be at least 3 times the hydraulic diameter d.
While a slim chamber with a l/d ratio of 8 is sufficient for a t2/t ratio of 2, t2/t must be higher if the chamber is stout. In addition the product of t2/t*l/d should be minimum of 8 and preferably 16. The average detention time t2 in a stout chamber with l/d=3 should be minimum 2.67*t for 8 or 5.33*t for 16. If e.g. T=70xc2x0 C. and the required minimum detention time according to regulations is t=0.5 hours, the average detention time in a chamber with l/d=3 should be minimum of 1.3 hours and preferably 2.7 hours.
The flow pattern within the chambers can also be influenced by thermal convection. If the incoming sludge has a slightly higher temperature than the sludge in the chamber, the incoming sludge stratifies at the top and moves down in a layer as it cools down. Because the flow has approximately a plug-flow characteristic in this case, the minimum detention time is only slightly shorter than the mean detention time. It is therefore an advantage to have a downward flow.
On the other hand, sludge particles that are heavier than the bulk of the sludge sink relative to the sludge bulk. Therefore it is also beneficial to provide an upward flow in one of the chambers. In an upward flow, the detention time of heavy particles is longer than the average detention time. The analog consideration applies for sludge particles that are lighter than the bulk of the sludge. They are slower if the flow through the chamber is directed downwards.
Consequently, it is beneficial if at least one of the chambers has an upward flow, and another chamber has a downward flow.
Heavy particles, such as grit, can accumulate at the bottom of a chamber if the upward velocity is slower than the sedimentation velocity of these particles. Therefore it is suggested to periodically remove grit from the bottom by withdrawing sludge from every chamber bottom. Connections to the sludge withdrawal pump and automatic valves are proposed for this reason. It is recommended to provide chambers with a steep bottom, e.g. a conical bottom, so that grit can slide down to the connection in the bottom. To prevent short circuits, sludge feeding should be interrupted for a time period t before grit is removed from the chambers.
Floating matter, such as grease, could accumulate at the sludge surface, particularly in chambers with downward flow. By raising the sludge level to the overflow pipe level, scum can be removed through overflow pipes. The overflow pipe also serves as an emergency overflow, in case the level control should malfunction.
To prevent recontamination of treated sludge through air connections, air should not flow from one chamber to another chamber. This can be prevented by continuously sucking air through the top of all chambers and blowing the air into the atmosphere. However, the exhaust air is very odorous. Therefore it is suggested to provide check valves and vacuum breakers at the top of every chamber. If the sludge level in a chamber drops, air enters through the vacuum breaker. If the sludge level rises, air is forced through the check valve to the atmosphere. It is further recommended to deodorize the exhaust air, e.g. in a bio-filter. The overflow pipes should be provided with water traps to prevent air leaving a chamber through an overflow pipe.
The incoming sludge flow can be heated to temperature T in a single step. It is also possible to heat it in two steps. It is beneficial to pre-heat the incoming sludge in a heat exchanger by simultaneous cooling of the effluent sludge. This serves for heat recovery and saves energy. In addition, depending on further sludge treatment, it can be necessary to cool the effluent sludge. The pre-heated sludge is then further heated to temperature T by another heating means, e.g. a hot water heat exchanger or by steam injection.
It is beneficial to have a high flow through the heating means. It is possible to re-circulate sludge through the heating means, whereby the incoming sludge is preferably blended with the re-circulated sludge before entering the heating means. It is also possible to use a high re-circulation flow to flush all heating means.